The present invention generally relates to the field of mineral processing for the recovery of gold from gold-bearing sulfide minerals and, more particularly, to mineral processing operations involving pressure oxidation of gold-bearing sulfide minerals to free gold to facilitate gold recovery.
Sulfide gold ores include gold contained in one or more sulfide minerals, such as for example pyrite, marcasite, arsenopyrite or pyrrhotite. Direct leaching of gold from these ores, such as direct cyanide leaching, typically results in only very low gold recovery. For this reason, these sulfide ores are often referred to as refractory sulfide gold ores. Recovery of gold from these refractory sulfide gold ores typically involves pretreatment of gold-bearing sulfide minerals to decompose at least a portion of the sulfide minerals to free the gold, thereby facilitating subsequent recovery of the gold by leaching the gold with a leach solution containing cyanide or some other lixiviant. The pretreatment may be performed, for example, on the whole ore, on a sulfide concentrate resulting from prior flotation operations, or on a blend of whole ore and ore sulfide concentrate.
Pressure oxidation is one pretreatment technique in which the ore and/or concentrate, is contacted with oxygen gas in an autoclave under high pressure to oxidize sulfide sulfur in the sulfide minerals thereby releasing gold for recovery. Typically, the sulfide sulfur is oxidized to a sulfate form. A typical pressure oxidation operation involves feeding particulate ore and/or concentrate slurried in water to the first compartment of a multi-stage, or multi-compartment, autoclave. Oxygen gas is fed to one or more of the compartments of the multi-compartment autoclave to effect the desired oxidation of sulfide sulfur for the purpose of freeing the gold for recovery. The gold remains in solid residue discharged from the autoclave following pressure oxidation, and the gold is recoverable from the residue by any suitable gold recovery technique, such as lixiviation of the gold with a cyanide, thiosulfate or other lixiviant for gold. As used herein, the terms xe2x80x9cmulti-stagexe2x80x9d and xe2x80x9cmulti-compartmentxe2x80x9d are used interchangeably in reference to an autoclave including one or more internal dividers separating the interior reactor volume within the autoclave into zones that progress in series in the general direction of flow through the autoclave, with each such divider acting as at least a partial barrier to flow between adjacent zones. The terms xe2x80x9cstagexe2x80x9d and xe2x80x9ccompartmentxe2x80x9d are used interchangeably herein to refer to such a zone within a multi-stage autoclave.
A significant expense associated with pressure oxidation is the cost of providing oxygen gas for use in the autoclave. There is often significant inefficiency in the use of oxygen gas and it is, therefore, common practice to feed to the autoclave a significant excess of oxygen gas over that stoichiometrically required for sulfide sulfur oxidation, with the excess oxygen gas being essentially wasted. Moreover, the oxygen gas is typically fed to the autoclave in a gas stream that is substantially enriched in oxygen compared to air. Providing such a purified stream of oxygen gas typically requires building and operating an oxygen plant to prepare an oxygen-enriched gas stream from air, such as for example by membrane or cryogenic separation techniques, which is expensive.
Another frequent problem with current pressure oxidation operations is thermal inefficiency in the autoclave. Oxidation of the sulfide mineral is exothermal, but maintenance of a minimum elevated temperature is required for adequate reaction kinetics. Therefore, in many instances heat, often in the form of steam, is added to the first compartment of a multi-stage autoclave to maintain an adequate temperature in the first stage, where the oxidation reaction is initiated. Conversely, in one or more subsequent stage of the autoclave, it is often necessary to add water to prevent the occurrence of excessively high temperatures, with the quantity of water required increasing as more steam is added to the first compartment.
The addition of steam in the front-end of the autoclave is undesirable because of the cost of generating the steam. Also, as the steam condenses in the autoclave it reduces the density of solids in the slurry and, therefore, the quantity of ore that may be processed through the autoclave per unit time. The addition of water in the back-end of the autoclave is likewise undesirable because the added water also tends to reduce the density of solids in the slurry, thereby further reducing potential ore throughout.
There is a need for pressure oxidation processes that more efficiently utilize oxygen gas fed to the autoclave and/or operate in a more thermally efficient manner.
The present invention generally relates to gold recovery operations and, more particularly, to a method and system for pressure oxidizing a gold-bearing mineral material feed to free gold from association with at least one sulfide mineral with which gold is associated.
Mineral material feed to the autoclave may be any gold-bearing material containing gold in association with at least one sulfide mineral, for which it is desirable to decompose at least a portion of the sulfide mineral to facilitate gold recovery. These mineral materials may be referred to as refractory sulfide materials, because at least a significant portion of the gold cannot generally be recovered by direct leaching of the mineral material with a lixiviant for gold, such as a cyanide or thiosulfate lixivant. The mineral material feed may include a whole ore, a sulfide concentrate prepared from prior flotation operations, or a blend of the two. Also, the mineral material feed may be or include tailings or other solid residue from prior mineral processing operations. With the present invention it has been found that autoclave performance, and especially oxygen gas utilization efficiency, is often significantly improved with the present invention.
A first aspect of the present invention generally relates to agitation of a mineral material slurry in an autoclave in which a pressure oxidation operation is being effected for gold recovery purposes. The Mineral material feed is introduced into the autoclave in a manner so that, in the autoclave, the mineral material is in a slurry, typically with water. Oxygen gas, typically under high pressure, is also introduced into the autoclave for use as an oxidant to oxidize at least a portion of the sulfide sulfur in the mineral material, thereby freeing at least a portion of the gold for possible subsequent gold recovery operations.
According to the first aspect of the invention, slurry present in the autoclave is agitated during pressure oxidation by at least one agitator disposed in the autoclave and operated to provide a pumping action in which portions of the slurry are continually drawn into and expelled from a cavity in the agitator. This pumping action is typically effected through rotation of at least a portion of the agitator in a manner to expel the slurry from the cavity in a generally radially outward direction, thereby creating a fluid suction within the cavity to draw additional slurry into the cavity for continuous cycling of slurry through the agitator while the rotation is continued.
Various refinements exist for features noted in relation to this first aspect of the present invention, and additional features may also be incorporated as well. These refinements and additional features may be incorporated individually or in any combination. In one refinement, the agitator has a fluid intake that is preferably axially aligned with a center of the noted cavity, and the agitator further has an axis of rotation that is aligned with the center of the cavity. Additional refinements involve directing a flow of the oxygen gas and/or the mineral material feed toward a fluid intake of the agitator through which slurry is directed to the cavity. In a preferred embodiment, one or both of these flows are directed in a vertically upward direction toward the fluid intake. In one embodiment, a flow of oxygen gas is introduced into the slurry within the autoclave from an oxygen gas supply line in substantially vertical orientation located below the fluid intake so that oxygen gas exiting the oxygen gas supply line flows in a substantially vertically upward direction toward the fluid intake, and the flow of mineral material feed is also directed in an upward direction toward the fluid intake, in a manner preferably designed so that the flow of mineral material feed intersects the corresponding flow of oxygen gas in the vicinity of the fluid intake. With these refinements concerning introduction of oxygen gas and mineral material feed, in combination with the pumping action of the agitator, dispersion of the oxygen gas through out the slurry and dissolution of the oxygen gas into the slurry liquid are promoted, with a result being that more efficient utilization of the oxygen gas to oxidize sulfide sulfur is typically achievable within the autoclave.
It should be appreciated that the pressure oxidation conducted in accordance with this first aspect of the invention will typically be performed in a multistage autoclave (also referred to interchangeably as a multi-compartment autoclave), with oxygen gas typically being introduced into each of the stages (or compartments). Moreover, and as will be discussed in more detail below in relation to a second aspect of the present invention, mineral material feed may advantageously be introduced into more than one of the autoclave stages to further enhance pressure oxidation performance.
In one embodiment of the first aspect of the invention, the cavity of the agitator is defined between a pair of spaced, typically vertically spaced, partitions of an agitator pump. In one embodiment, the first and second partitions are disposed in at least substantially horizontal relation, with the entire first partition being disposed at a lower elevation than the entirety of the second partition. Other orientations could possibly be utilized for the first and/or second partitions. An aperture is formed in the first partition such that slurry in the autoclave is drawn into the cavity of the agitator through the aperture. The aperture may be the fluid intake of the agitator through which slurry is drawn to supply the cavity. In one refinement, however, the agitator includes a pump inlet conduit or the like to provide a flow path through which slurry is drawn to be directed through the aperture and into the cavity. In this case, the fluid intake to of the agitator would be an open end of the pump inlet conduit, or the like, through which slurry is initially drawn into the agitator for fluid communication to the cavity. In one preferred embodiment, the pump inlet conduit projects at least generally downwardly toward the bottom of the autoclave in an at least substantially vertical orientation and/or in axial alignment with the direction in which the flow of oxygen gas is introduced into the slurry in the autoclave. Other configurations for the fluid intake of the agitator are also possible. Moreover, whether or not the agitator includes a pump inlet conduit, or the like, in a further refinement the oxygen gas is preferably introduced into the autoclave at a location which is xe2x80x9cclosexe2x80x9d to the fluid intake of the agitator. In one embodiment, the spacing between a discharge end of an oxygen inlet line and the corresponding fluid intake of the agitator is no larger than about 6 inches.
As noted, a pumping action of the agitator, wherein slurry is continuously drawn into and expelled from the cavity during operation of the agitator, is typically effected by rotation of at least a portion of the agitator. For example, the agitator may include a plurality of vanes that are rotated within the slurry in the autoclave during pressure oxidation. The vanes typically extend in a direction generally radially outward and away from the cavity. When rotated about an axis of rotation extending substantially through the center of the cavity, the vanes help to expel fluid from the cavity in a generally radially outward direction and create a fluid suction to draw additional slurry into the cavity, thereby creating a pumping action. Shear at the outward edges of the rotating vanes, in combination with the pumping action of the agitator, is believed to enhance effective dispersion of the oxygen gas throughout the slurry to promote efficient use of oxygen to oxidize sulfide sulfur. In one embodiment, the vanes are incorporated into the agitator so that each vane extends in a vertical direction at least between the first and second partitions (i.e., a portion of each vane may extend vertically beyond the first and/or second partition) and in a generally radially outward direction beyond the perimeter of the first partition and/or second partition. The vanes may each extend generally radially inward of a perimeter of the inlet aperture within the first partition in one embodiment (i.e., a portion of each vane may be disposed xe2x80x9coverxe2x80x9d the inlet aperture). Alternatively, the vanes may each terminate in a generally radially inward direction at a location not within a perimeter of the inlet aperture (i.e., no portion of the vanes is disposed xe2x80x9coverxe2x80x9d the inlet aperture).
A second aspect of the present invention generally relates to the manner in which mineral material feed is introduced into an autoclave for pressure oxidation, with mineral material feed being separately introduced at at least two different locations within the autoclave. At least a first flow of mineral material feed, typically in a slurry with water, is introduced into the autoclave at a first location (e.g., into a first compartment of a multi-stage autoclave). At least a second flow of mineral material feed, also typically in a slurry with water, is introduced into the autoclave at a second location, which is spaced from the first location (e.g., into a second compartment of a multi-stage autoclave). In a further embodiment of this second aspect of the invention, a flow of oxygen gas is directed into each of the compartments of a multi-stage autoclave. In any case, slurry within the autoclave is agitated to enhance the oxidation process, preferably with each compartment of the autoclave being independently agitated by at least one agitator disposed within each compartment.
Various refinements exist for features noted in relation to this second aspect of the present invention and additional features may also be incorporated as well. These refinements and additional features may be incorporated individually or in any combination. In one refinement, at least one of, and more preferably both of, the first and second flows of mineral material feed, are introduced into the autoclave in an at least a generally upward direction (e.g., such that the flows are projected at an upward angle). Also, significant refinements are achievable through a combination of using the agitation of the first aspect of the invention in the vicinity of at least one, and preferably both, of the first and second flows of mineral material feed. For example, the agitation of the first aspect, optionally including any refinements thereto, may be advantageously implemented in first and second compartments of a multi-stage autoclave in combination with the split mineral material feed of the second aspect of the invention. Portions of total mineral material feed to the autoclave may be allocated between the first and second flows of mineral material feed in any desired manner. However, for enhanced performance it is generally preferred that at least about 25% of the total mineral material feed to the autoclave be allocated to each of the first and second flows, and more preferably at least about 50% of total mineral material feed to the autoclave is allocated to the first flow. In one embodiment, the total mineral material feed is split approximately equally between the first and second flows.
As noted, the autoclave will often be a multi-stage autoclave. Such an autoclave has a plurality of stages, or compartments, arranged in series. One preferred autoclave for use with the present invention includes four compartments. Flow in such a multi-compartment autoclave proceeds from the first compartment in series to the second compartment in series, and so on through the last compartment in series. The oxidized slurry is then typically discharged from the last, or most downstream, compartment. These compartments are at least partially isolated from each other by a divider disposed between adjacent compartments in series. In one embodiment, slurry moves from one compartment to the next succeeding compartment in series by overflowing the divider or passing through a restricted opening through or adjacent to the divider. The divider is typically a wall or other partition, such as of metal construction. In one embodiment of the invention, the first flow of mineral material feed is introduced into the first compartment in series and the second flow of mineral material feed is introduced into the second compartment in series.
During conventional pressure oxidation of sulfide gold ores, the mineral material feed is introduced into only the first in series of the compartments. Oxygen gas is typically added to each of the compartments and oxidation of sulfide minerals proceeds to a greater extent as the slurry moves from compartment to compartment through the autoclave. In a preferred embodiment of the present invention, however, mineral material feed is introduced into at least each of the first and the second compartments in series. This has been found advantageous from both the perspectives of thermal efficiency and oxygen utilization. In conventional pressure oxidation of sulfide gold ores, when mineral material feed is introduced only into the first compartment, heat generated in the first compartment from the exothermic oxidation reaction is not sufficient for autothermal operation, and it is therefore often necessary to add heat, typically in the form of steam, to the first compartment to maintain the first compartment at a sufficiently high temperature. In later compartments, however, as the oxidation reaction progresses, cooling is often required to avoid excessive temperatures. Such cooling is often accomplished by adding water in one or more of the later compartments. The effect of adding steam in the first compartment and water in later compartments is that the density of the slurry in the autoclave is reduced, and therefore also throughput of mineral material is reduced.
By splitting the mineral material feed between the first compartment and the second compartment according to the second aspect of the invention, less steam is required in the first compartment due to a reduction in feed to the first compartment that must be heated to reaction temperature. Furthermore, the retention time in the first compartment is increased, which in turn increases the extent to which sulfides are oxidized in the first compartment, resulting in higher heat production in the first compartment per unit of mineral material feed to the first compartment and further reducing steam requirements in the first compartment. Hot slurry flowing from the first compartment into the second compartment often provides sufficient heat to maintain the temperature in the second compartment at the desired elevated reaction temperature, even with the introduction of the second flow of mineral material feed into the second compartment. In some instances, it may be desirable to provide some supplemental heating from an outside heat source, such as by steam addition. Even if such supplemental heating is required in the second compartment, the total supplemental heat to the autoclave will typically be significantly lower than required in the conventional situation, in which mineral material feed is introduced into only the first compartment. Performance in specific instances will depend, of course, upon the sulfide sulfur content of the mineral material being processed and the specific conditions under which the pressure oxidation is being operated. An additional benefit from splitting total mineral material feed between first and second compartments is that it is often also possible to reduce water additions to prevent excessive temperatures in downstream compartments. Furthermore, the enhanced thermal characteristics in the first and second compartments are believed to promote efficient utilization of oxygen gas fed at the autoclave.
In a particularly preferred implementation of the invention, the first and second aspects are combined. For example, mineral material feed may be introduced into the first two compartments of a multi-stage autoclave according to the second aspect of the invention, and the agitation of the first aspect may beneficially be implemented in the first and/or second compartments, and optionally also in other compartments. By reducing steam and water additions by splitting the mineral material feed between compartments, it is typically possible to process a higher density slurry through the autoclave, while the pumping agitation promotes efficient utilization of oxygen gas to adequately oxidize sulfide minerals in the higher density slurry.
With each of the first and second aspects of the present invention, oxygen gas is typically introduced into multiple compartments, and preferably into each of the compartments of a multi-stage autoclave. However, a greater portion of total oxygen gas fed to the autoclave is typically introduced into each compartment into which mineral material feed is introduced, and a lesser portion is introduced into each compartment in which no mineral material feed is introduced. In the case of the second aspect of the invention, when the mineral material feed is split between compartments, it is preferred that the relative quantities of oxygen gas introduced into each of those compartments be approximately in proportion to the relative quantities of mineral material feed introduced into each of those compartments. For example, when 50% of the total mineral material feed is introduced into each of the first and second compartments, oxygen gas feed to each of the first and second compartments should be approximately equal, with perhaps about 45% of the total oxygen gas being fed to each of those compartments and the third and fourth compartments each receiving perhaps only 5% or less of the total oxygen gas.
A third aspect of the present invention generally relates to the manner in which the slurry is agitated within a pressure oxidation autoclave in a gold processing system. This system includes a mineral material feed system for providing a mineral material feed to an autoclave, and an appropriate gold recovery system to receive autoclave discharge from the autoclave for the purpose of recovering gold from solid residue of the autoclave discharge (e.g., via cyanide, thiosulfate or other leaching of the gold). The autoclave includes a pressure vessel having a plurality of fluid connections. There is at least one mineral material feed inlet for introducing mineral material feed into the pressure vessel and at least one discharge outlet for discharging oxidized slurry from the pressure vessel. The autoclave also includes at least one oxygen gas feed inlet for introducing oxygen gas into the pressure vessel from an appropriate oxygen supply system.
The autoclave of this third aspect of the invention also includes at least one agitator pump, of the type previously noted with respect to the first aspect of the invention, disposed at least partially inside of the pressure vessel. This agitator pump typically includes a drive shaft, a pair of vertically spaced first and second partitions, and a plurality of vanes. Preferably, at least a portion of at least one, and more preferably each of, these vanes interfaces with and extends between the first and second partitions. The first partition includes at least one inlet aperture which effectively defines an inlet to the space between the first and second partitions, or stated another way this inlet aperture defines a pump inlet.
Various refinements exist for the features noted in relation to this third aspect of the present invention and further features may also be incorporated in this third aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The autoclave may have multiple stages. Preferably, at least one agitator pump is used in each of these stages. By including multiple stages, the various combinations of features discussed above in relation to the second aspect of the present invention may be used in combination with this third aspect of the present invention as well (e.g., simultaneously introducing the mineral material feed into at least two different compartments of a multi-stage autoclave).
The vanes associated with the third aspect of the invention need not be confined to the space between the first and second vertically spaced partitions. For instance, the vanes may extend radially beyond a perimeter of one or more of the first and second partitions, the vanes may extend vertically beyond the first partition in a direction which is at least generally away from the space between the first and second partitions, the vanes may extend vertically beyond the second partition in a direction which is at least generally away from the space between the first and second partitions, or any combination thereof. Any portion of the vanes which extends vertically beyond the first and/or second partition in a direction which is at least generally away from the space between the first and second partitions may be disposed entirely radially beyond a perimeter of the adjacentmost partition or may be disposed in at least partial vertical alignment with the adjacentmost partition (e.g., by disposing the first and/or second partition within a notch or the like which is formed in the vanes).
The spacing between the first and second partitions may be characterized as defining a pump cavity, of sorts, and which is accessed by the inlet aperture in the first partition. In one embodiment the first and second partitions are each disposed in at least substantially horizontal relation, with the first partition being disposed at a lower elevation than the second partition. The inlet aperture in this case projects toward a bottom of the pressure vessel. In one embodiment a pump inlet conduit interfaces with the first partition in alignment with the first inlet aperture and extends at least generally downwardly toward the bottom of the autoclave, preferably in at least substantially vertical relation. In any case, oxygen gas that is introduced into the pressure vessel is preferably directed at least generally upwardly and toward a fluid intake of the agitator. More preferably, an oxygen gas supply line is axially aligned with a center of the inlet aperture and/or the inlet conduit in a vertical orientation. Similarly, preferably the mineral material feed that is being introduced into the pressure vessel of the autoclave is directed at least generally upwardly and toward the fluid intake of the agitator. Disposing an oxygen gas discharge line and a corresponding mineral material feed discharge line sufficiently close to a fluid intake further promotes intimate mixing of oxygen gas and mineral material via the agitator pump. In one embodiment, a vertical spacing between the discharge end of the oxygen gas supply line and the fluid intake of the agitator is preferably more than about 6 inches.
A fourth aspect of the present invention generally relates to the manner in which mineral material feed is introduced into a pressure oxidation autoclave in a gold processing system. This system includes a mineral material feed system for providing mineral material feed to a pressure oxidation autoclave and an appropriate gold recovery system to receive autoclave discharge for purposes of recovering gold from solid residue of the autoclave discharge (e.g., via cyanide, thiosulfate or other leaching of the gas). The autoclave includes a pressure vessel having at least two compartments. At least one agitator is disposed in each of first and second compartments. There is at least a first mineral material feed inlet for introducing mineral material feed directly into the first compartment of the pressure vessel, and there is at least a second slurry feed inlet for introducing mineral material feed directly into the second compartment of the pressure vessel, preferably simultaneously with the introduction of mineral material feed into the first compartment. There is also at least one discharge outlet for discharging oxidized slurry from the pressure vessel to be received by the gold recovery system. The pressure vessel also includes at least one oxygen gas inlet for introducing oxygen gas into the pressure vessel from an appropriate oxygen gas supply system.
Various refinements exist for the features noted in relation to this fourth aspect of the present invention and further features may also be incorporated in fourth aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The mineral material feed may be introduced into one or both of the first and second compartments in a generally vertically upward direction of flow, generally directed at a corresponding agitator. In one embodiment, a first mineral material feed supply line extends within the pressure vessel along at least a portion of the bottom thereof and includes a discharge end portion that extends away from the bottom to direct the first flow of mineral material feed into the first compartment, and a second mineral material feed supply line extends within the pressure vessel along at least a portion of the bottom thereof and includes a discharge end portion that extends away from the bottom to direct the second flow of mineral material feed into the second compartment. Any design for providing the first mineral material slurry to each of these first and second mineral material feed supply lines may be utilized. For instance, a single main supply line could penetrate the pressure vessel and be directed along the bottom portion of the pressure vessel and the first and second mineral material feed supply lines could extend upwardly therefrom. Also, those various combinations of features discussed above in relation to the third aspect of the present invention may also be utilized by the subject fourth aspect of the present invention.
A fifth aspect of the present invention generally relates to the manner in which mineral material feed is introduced into a pressure oxidation autoclave in a gold processing system. This system includes a mineral material feed system for providing mineral material feed to a pressure oxidation autoclave and an appropriate gold recovery system to receive autoclave discharge for purposes of recovering gold from solid residue of the autoclave discharge (e.g., via cyanide, thiosulfate or other leaching of the gold). The autoclave includes a pressure vessel having at least one compartment, and preferably at least two compartments. At least one agitator is disposed in each such compartment of the autoclave. There is at least one mineral material feed inlet for introducing the mineral material feed into the autoclave in a generally upward direction, preferably directed generally upward in a direction toward an agitator. There is also at least one discharge outlet for discharging oxidized slurry from the pressure vessel to be received by the recovery system. The pressure vessel also includes at least one oxygen inlet for introducing oxygen into the pressure vessel from an appropriate oxygen supply system for effecting the pressure oxidation operation.
Various refinements exist for the features noted in relation to this fifth aspect of the present invention and further features may also be incorporated in the subject fifth aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. Those various combinations of features discussed above in relation to the third aspect of the present invention may be utilized by the subject fifth aspect of the present invention. Similarly, those various combinations of features discussed above in relation to the fourth aspect of the present invention may be utilized by the subject fifth aspect of the present invention as well.
Moreover, any of the features of any of the first, second, third, fourth and fifth aspects of the invention discussed herein may be combined in any compatible combination with any other of such features.